The present invention relates to electrical circuits which detect a predefined reoccurring characteristic or point in the waveform of a periodic signal; and particularly to such circuits which detect zero crossings of the periodic signal, and more particularly to only the zero crossing at the beginning of each signal cycle.
It often is desirable to be able to detect a given reoccurring characteristic, such as a maximum, minimum or zero crossing point, of a time periodic electrical signal. For example, a sinusoidal signal has two zero crossings per cycle, i.e. one crossing at the beginning of the cycle and another at the mid point of the cycle; of course there is another zero crossing at the end of the cycle when another signal cycle begins. Detecting one zero crossing per cycle can be employed to determine the frequency or the period of the signal. It also is beneficial to detect the occurrence of corresponding characteristics in two different signals in order to determine the phase relationship of those signals.
Circuits have been developed to detect zero crossings of a signal. However, such prior circuits detected every zero crossing and could not distinguish the zero crossing at the beginning of a cycle from the one at the cycle mid point.